In recent years, fingerprints have played an important role in a field known as biometrics. Biometrics refers to the field of technology devoted to identification of individuals using biological traits. Fingerprints may be acquired in the form of digital images that are amenable to processing by computer software. There are a number of existing techniques for capturing a fingerprint in a digital format, including capacitive sensors, thermal sensors, and optical sensors.
Capacitive sensors have problems with electrostatic discharge (ESD), and with sensing dry fingers. Thermal sensors use a heater and correspondingly have an undesirable heat up lag time. Optical sensors do not use a heater, typically have less of a problem with ESD than capacitive sensors, and are better able to sense a dry finger. But current optical sensors have a problem with distinguishing between a real human finger and a “fake” finger, such as a photocopy of a real human finger.
Fingerprint sensors can be classified as either area sensors or swipe sensors. Swipe sensors are also referred to as linear sensors, since they typically use a linear array, as opposed to an area array, of sensing elements. With an optical area sensor, images of a finger are captured while the finger is held stationary on the area array of sensing elements. With an optical swipe sensor, a user slides or swipes his finger on the sensor, which captures multiple images during the swipe. The sequence of captured images is then combined or “stitched” together to form one continuous fingerprint image.
Area sensors typically take up more space than swipe sensors. In some applications, such as for some portable electronic devices (e.g., cellular telephones, personal digital assistants (PDA's), laptop computers, etc.), it is undesirable to have a large area dedicated to placing a finger on for the purpose of finger recognition.